In the external beam radiotherapy treatment of cancer, it is important to plan the treatment so that the irradiated energy the tumor is sufficiently high while the dose level at the normal tissue is made as small as possible. In planning a treatment with multiple beams, it is necessary to optimize the position and the intensity of the sources of radiation. In this study, we proposed a treatment planning method by using Mixed Integer Programming (MIP), which can control the number of beams, comsidering the operational feature of the treatment. The effectiveness of the proposed method is demonstrated by an illustrative case study.